1. Field of the Invention
The present invention relates to an actuator system having an output shaft which is held in a first position by a spring and upon command an electric motor drives the shaft to a second position against the force of the spring and holds the shaft in the second position until a further command allows the spring to return the shaft to the first position.
2. Description of Co-Pending Application
In a co-pending patent application of Christopher M. Lange entitled “Drive Circuit And Method For An Electric Actuator With Spring Return” Ser. No. 08/904,005, Filed Jul. 31, 1997, issued Jun. 19, 2001 as U.S Pat. No. 6,249,100B1 and assigned to the assignee of the present invention, an actuator system is shown in which an output shaft is positioned by a spring in a first position and upon command, is driven through a gear train to a second position by an electric motor. The circuit includes an input circuit for providing a current large enough to rotate the shaft against the force of the spring. A rotation sensor produces a signal when the motor and shaft are stalled at the second position and a modulation circuit receiving the signal from the rotation sensor operates to reduce the current to the motor so as to hold the stalled shaft and motor at the second position with a minimum of energy usage. In some applications, this circuit is intended for use in high ambient temperature conditions and accordingly, the rotation sensor utilizes a Hall effect device known to be able to withstand high temperatures. The use of a Hall effect device involves some undesirable features because it is undesirably complicated to implement and is slower than desired. The increased current to the motor at the stalled condition produces an increased torque that can over stress the gear train, at least temporarily, and gear damage may result.